Wireless communication of signals typically involves the use of defined bands of frequency spectrum from which a carrier signal or signals are utilized. Frequency bands utilized by many wireless communication systems are relatively narrow, allowing antennas to be tuned to resonate at a particular frequency for reception and/or transmission of signals within the relatively narrow frequency band of the system. Such antennas generally do not provide good wideband frequency response.
Various wideband antenna configurations have been developed in the past for specific uses, such as military and space applications including radar. For example, tapered slot, horn, spiral, conical, log periodic and planar circular monopole antennas have been utilized in wideband communications.
The tapered slot antenna was first introduced in 1974 and was later improved in 1979 to employ an exponential taper configuration, giving better broadband impedance matching. Exponential taper configurations of a taper slot antenna, generally referred to as Vivaldi aerials, are shown in FIGS. 1A-1C. These antenna configurations provide wideband characteristics, delivering high gain with a directive radiation pattern.
As can be seen in FIGS. 1A-1C, the tapered slot antenna physical structure is “blade” like, wherein cathode (shown as element 101 in FIG. 1A) and anode (shown as element 102 in FIG. 1A) conductors are disposed in a plane having a tapered slot therebetween. The tapered slot acts as a waveguide to setup the fields for efficient radiation. A signal input/output is provided at,the tapered slot end (designated R in FIG. 1B) and the antenna aperture (designated A in FIG. 1B) is defined by the taper of the slot.
As can be seen in FIGS. 1A-1C, the tapered slot antenna includes two regions; a setup region and a flare region. The antenna design usually requires a long setup region to give directivity, resulting in tapered slot antennas which are generally relatively long in the axial direction. Accordingly, the antenna length (designated L in FIG. 1B) is typically in the range of 2λo<L<12λo, where λo is the free space wavelength of the lowest resonance frequency of the antenna. Such a relatively long antenna configuration can be useful in providing very clean polarization. However, the space required for such long antenna configurations makes the antenna characteristics more sensitive to placement and, hence, limited application in various mobile communication or other systems.
The width of the aperture (A) determines the lowest resonance frequency (i.e., A≧λ0/2, where λ0 is free space wavelength of the lowest resonance frequency). However, there is often a problem with lower frequency termination. Specifically, as shown above, the aperture is the half wave length of the lowest resonate frequency of the antenna and, at this frequency, the antenna is not well matched because currents are not terminated properly. As can be appreciated from the foregoing, tapered slot antennas provide poor matching characteristic for lower operating frequencies, where flare aperture of the antenna is at its maximum.
Impedance of a tapered slot antenna is not constant over a large frequency range. Accordingly, an optimized taper may present a “self-similar” like condition to the current vector launched within the slot. An imbalance resulting in unsymmetrical current flow will also degrade the propagation of certain frequencies, thereby reducing broadband performance and radiation efficiency. Accordingly, tapered slot antennas utilize balanced feed systems to ensure radiation patterns are controlled. For example, a cathode and anode feed are typically implemented for aperture radiation equivalent to a dipole, thus requiring a balanced feed mechanism.
Antipodal Vivaldi aerial configurations have been developed in an attempt to provide more balanced fields. FIG. 1C shows an antipodal Vivaldi aerial configuration. Although providing improvement with respect to balanced fields, such antenna configurations still suffer from the other disadvantages associated with Vivaldi aerial configurations discussed above.
Planar circular monopole antennas comprise a disk shaped plate as a monopole providing omni-directional communications. An example of a planar circular monopole antenna is shown in FIG. 2, wherein disk shaped plate 201 is disposed orthogonal to ground plane 202. The use of such antennas is typically limited to indoor use.
The design of planar circular monopole antennas typically provides very broadband communication. However, at the higher operating bands, the radiation begins to experience substantial multi source contribution. Accordingly, the radiation pattern associated with a planar circular monopole antenna starts to deteriorate at these frequencies. Accordingly, the operating frequencies for such antennas are effectively limited by the radiation pattern being deteriorated to roughly a couple of wavelengths above the lowest frequency the antenna is designed for.
According to the planar circular monopole antenna design, the height of the disk is typically sized to correspond to the quarter wave length of the lowest frequency the antenna is designed for. Accordingly, the size of planar circular monopole antennas are typically relatively large. Moreover, at this lowest frequency, the impedance is not well matched because of current termination.
Broadband parallel plate antennas, shown in detail in U.S. Pat. No. 5,748,152 issued to Glabe et al., the disclosure of which is hereby incorporated herein by reference, provide a slot antenna element on a substrate material having a conductive plate thereover. As shown in FIG. 3, slot 310 comprises two flared slot sections 311 and 312 which are extended towards the back of the flare in both cathode 301 and anode 302, respectively. These slots are filled with absorptive material, primarily to minimize the overall aperture dimensions as well as to provide a better current termination. This antenna provides a relatively complex antenna configuration requiring additional manufacturing cost and larger antenna size.